1. Field of the Invention
This invention relates to circuit breakers, and particularly to a circuit breaker with an inertia device which prevents tripping of the circuit breaker by shock loading.
2. Background Information
Multipole circuit breakers commonly have a trip unit that monitors each pole for currents exceeding certain current/time characteristics. The response of the trip unit to an overcurrent in any pole is coupled to a single spring powered operating mechanism through a trip bar mounted for rotation about its longitudinal axis. A latch arm on the trip bar unlatches the spring powered operating mechanism when rotated by the trip unit. The latch is designed such that a relatively light force applied to the trip bar is adequate to release the relatively strong force stored in the spring powered operating mechanism. Such an assembly can be susceptible to xe2x80x9cshockoutxe2x80x9d or unintentional tripping of the circuit breaker in response to mechanical shock. The resultant nuisance trips are unacceptable in many applications.
There is a need therefore for an improved circuit breaker that is not unduly susceptible to shock loads.
This need and others are satisfied by the invention which is directed to a circuit breaker incorporating one or more inertia devices that restrict movement of trip bar in response to shock loading but do not interfere with normal operation of the trip bar in unlatching the operating mechanism. More specifically, the circuit breaker includes a casing, separable contacts, and an operating mechanism incorporating a latch and that opens the separable contacts when the latch is unlatched. The circuit breaker further includes a trip assembly comprising a trip bar unlatching the latch when moved to a trip position and a trip device moving the trip bar to the trip position in response to selected conditions of current flowing through the separable contacts. An inertia device prevents movement of the trip bar to the trip position in response to a mechanical shock. The trip bar comprises an elongated member mounted for rotation about a longitudinal axis. The inertia device comprises at least one mass and a compliant mount mounting the mass adjacent the trip bar to engage the trip bar in response to the mechanical shock and prevent unlatching of the latch. The compliant mount is a spring mount which comprises a mounting support, a mounting rod carrying the at least one mass and slideable relative to the mounting support and a spring coupling the mass to the mounting support. The spring can be a helical compression spring mounted coaxially on the mounting rod. The trip assembly can include a trip housing in which the elongated member is rotatably mounted and to which the mounting support is mounted.
A pair of the masses can be provided, each mounted by a compliant mount on opposite sides of the trip bar to counteract shock loading in opposite directions.